1. Field of the Invention
The present invention pertains to an apparatus for welding thermoplastic material with a laser. More particularly, the apparatus is for affixing an interlocking fastener at a predesignated position (for example, at the open end or folded end of a folded, extruded thermoplastic material, or between two separate sheets of thermoplastic material) in the manufacture of a resealable plastic bag.
2. Description of the Related Art
To fabricate products from sheets of thermoplastic material, it is often necessary to seal the thermoplastic material to itself or to similar material. Such materials are heat fusible, and it is therefore possible to form the seals (or welds) by the application of thermal energy.
As is known, the temperature to which the thermoplastic sheet is raised to form the weld must be carefully controlled. If the temperature is raised too far, the thermoplastic material melts and simply separates. On the other hand, if the temperature is not raised far enough, no weld is formed. Therefore, in forming a weld, it is necessary to raise the temperature of the thermoplastic material to a point between its softening temperature and the melting point thereof.
Conventional techniques for raising the temperature include steps such as mechanically pressing two sheets of thermoplastic film together between heated sealing bars, applying radio-frequency or ultrasonic energy, or applying heated compressed air blown through opposed nozzles to seal the material together. Such techniques, however, have significant drawbacks, primarily in the speed at which the thermoplastic material may be welded.
In view of this shortcoming, it has been recognized to use a laser to form a weld in sheets of thermoplastic material. For example, U.S. Pat. Nos. 3,560,291 and 3,769,117 describe bonding of thermoplastic resin films using radiation from a laser source by focusing the laser beam to a point and scanning or moving the laser beam relative to the thermoplastic material. Additionally, U.S. Pat. No. 4,069,080 describes welding thermoplastic sheets together by focusing a laser beam into a line, thereby eliminating the need for relative movement between the laser beam and the thermoplastic sheet.
While these patents describe systems which are advantageous from the perspective of utilizing a laser beam to form the weld rather than mechanical heating means, the state of the art is still not acceptable.
For example, the welds formed by such systems are often incomplete due to various combinations of improper registration between the sheets, improper coordination of laser power, improper conveying speed of the sheet, and incorrect sealing pressure. Incomplete welds (sometimes called "peel welds") are also caused by absorption of the laser power by the first layer of thermoplastic material, thereby reducing the amount of laser power available to heat the second layer of plastic material. This problem is particularly acute in devices where more than two layers of thermoplastic material must be welded, thereby precluding the simple solution of projecting laser power from both sides of the weld. Furthermore, incomplete welds may be caused by an improper distribution of the laser power within the profile of the layers of thermoplastic material.